In a system, circuit blocks may be operated under different voltage supply levels. For example, the Micro Control Unit (MCU) voltage supply level could be 1.2V, the Dynamic Random Access Memory (DRAM) voltage supply level could be 1.5V, and the NOR flash voltage supply level could be 3V. However, the operation of a circuit block is not guaranteed when the supplied voltage supply is lower than expectation. For example, a system error could occur when one circuit block is still delivering/accessing data to another block supplied by a voltage supply lower than expectation.